In a case that has shocked Hong Kong and inflamed tensions in a city now in its third week of mass pro-democracy protests, six police officers have been caught on video kicking and beating a prominent political activist.

The man allegedly assaulted was Civic Party member and social worker Ken Tsang, who was one of 45 people arrested early Wednesday as demonstrators attempted to throw up fresh barricades across a major thoroughfare leading to the main financial district.

Mabel Au, local director at Amnesty International, said there was little doubt of “excessive force” after Tsang was filmed being “taken to a dark corner” and “kicked and beaten up by the police for four minutes” with hands secured behind his back. “We are very shocked and disappointed by such behavior,” she said.

Video of the attack has been repeatedly broadcast on local television news and the officers involved have been assigned to other duties.

A spokesman for Tsang told TIME the police’s actions were “clearly criminal” and reassignment was not enough. “They should be arrested,” he said.

A police statement early Wednesday expressed “concern” over the video and promised that the police would conduct an investigation “impartially.”

Trouble sparked shortly before 10 p.m. local time on Tuesday, when several dozen demonstrators stopped traffic at the Lung Wo Road tunnel, a key artery that runs by Hong Kong’s government headquarters and parallel to the main protest site.

After attempting to intervene, some 30 police officers became trapped in the tunnel, hemmed in by protesters on either side. Scuffles broke out and the police retreated.

Protesters then set about reinforcing defenses. Hollow median dividers were filled with water and steel railings intertwined with cable ties, car tires and plastic wrap. Concrete blocks were hauled out of the tunnel’s gutter and secured by steel wire to block the roadway. Meanwhile, hundreds gathered on the lawns of Tamar Park, beside the shimmering waters of Victoria Harbour.

Demonstrators also built a symbolic grave for the head of the city’s government, Chief Executive (CE) Leung Chun-ying, also known as “C.Y.” Protesters are demanding the 60-year-old resigns and his successor chosen by free elections in 2017. The central government in Beijing insists that it must screen all candidates first.

“Everyone wants C.Y. to step down, but if that’s all that happens the next [CE] will be just the same and nothing will change without first changing the political system,” says Angel, a 30-year-old protester.

An uneasy calm held over Lung Wo Road until around 3 a.m., when hundreds of police brandishing batons and pepper spray bore down to clear the area. Davis Matthews, 27, showed TIME video footage of an officer firing pepper spray into his face.

“I wasn’t protesting anything, or shouting, but just documenting what was going on,” he said. “It was like a military action. We made eye contact just before he sprayed me and he didn’t seem happy.”

Police and legislators insist the demonstrations are an issue of law and order, and that officers are simply reclaiming public roads. Supporters of the democracy movement insist the conflict—now the most politically significant protest in China since the Tiananmen occupation of 1989—can only be solved by dialogue.

“We are eager, we are happy to engage in dialogue, but they turn us down,” pro-democracy lawmaker Emily Lau told a Hong Kong Foreign Correspondents Club luncheon Tuesday. “The way out is for the government to have talks.”

However, the potential for meaningful negotiation is hindered by a lack of leadership. The democracy movement is comprised of a disparate collection of students, liberal politicians and activist groups—and protest actions, such as last night’s attempt to barricade Lung Wo Road, are happening spontaneously.

Alex Chow, leader of the Hong Kong Students’ Federation, admitted Wednesday that the previous evening’s foray “wasn’t the students” at all. “It was an action launched by people discussing it online … launched by the citizens,” he said.

Fred Choi, 35, a radio engineer speaking by the westernmost barricades on Lung Wo Road, told TIME “We are not [from any of the main political groups] but independents who care about democracy.”

On Thursday, Chief-Executive Leung—whose approval rating has dropped to an all-time low of 42%—is due to address the Legislative Council, but students have pledged to block his path, giving renewed potential for clashes with police.

Many ordinary citizens are becoming frustrated by the continued disruption caused by the protests. The city’s subway is at breaking point, as commuters try to find alternatives to taking motor transport through the protest areas. Retail businesses near the protests are also hard hit.

However, the video of police officers apparently assaulting a peaceful demonstrator will galvanize support for the protesters, who have planned a large demonstration outside the city’s police headquarters on Wednesday afternoon.

Kai Ming Wong, a 43-year-old engineer, tells TIME he couldn’t focus on work after hearing about the police violence. “It’s ridiculous,” he said. “How are we going to trust the police in the future? It’s never been like this in Hong Kong before.”